1. Field of the Invention
The present invention relates to a charging device used for an image forming apparatus such as a copying machine, a printer, or facsimile machine.
2. Description of the Related Art
An electrophotographic image forming apparatus forms a toner image on a charged photosensitive member. As an example of a device that charges a photosensitive member, there is a corona charger that performs corona discharge. The corona charger generates a discharge product such as ozone O3 or nitrogen oxide NOx because the photosensitive member is charged by using the corona discharge.
When the discharge product generated by using the corona discharge is adhered to the photosensitive member, moisture from the air is absorbed and surface resistance is thus reduced. In particular, under a high-humidity environment, an electrostatic latent image corresponding to image information cannot be accurately formed at the adhesion portion of the discharge product (this problem is referred to as “image deletion”).
Japanese Patent Application Laid-Open No. 2008-046297 discusses a configuration in which an opening of a corona charger is covered with a shutter to prevent the deposition of a discharge product to a photosensitive member when an image is not formed to solve the “image deletion”. Specifically, the configuration is discussed that the shutter is moved for opening/closing along the longitudinal direction of the corona charger.
To solve the “image deletion”, there is a method for preventing the moisture absorption of the discharge product by heating the photosensitive member or a method for removing the discharge product by polishing the photosensitive member. On the other hand, advantageously, with the arrangement of the shutter to the corona charger, energy required for heating is suppressed (energy saving) and a polishing amount of the photosensitive member is further suppressed, thereby extending the life of the photosensitive member.
The corona charger is arranged adjacently to the surface of the photosensitive member. Therefore, the shutter has to be arranged in a narrow gap. It is not preferable for the photosensitive drum to be rubbed by the shutter because the photosensitive member is damaged.
Then, as a result of dedicated study by the inventor, by disposing a holding member for regulating the variation in a sheet-like member at the edge of a sheet, the shutter could be provided in a narrow gap. Further, to prevent sagging-down of a sheet-like shutter along the longitudinal direction of the corona charger, it became apparent that it was preferable to apply urging force to the shutter with a sheet winding member along the longitudinal direction of the corona charger.
A charging wire of the corona charger needs to be replaced when it has been used for an operating life. However, the charging wire cannot be replaced in the corona charger having the shutter. Specifically, the holding member for suppressing the variation in sheets is disposed at the edge of the sheet-like shutter member, and the charging wire can be therefore replaced only by removing the shutter or setting the charging wire underneath.